Evaluation of the Use of Visible Implant Tags in Age-0 Atlantic Cod

Abstract

The visible implant elastomer (VIE) tag and the soft visible implant alphanumeric (VIAlpha) tag were evaluated as methods for tagging age-0 Atlantic cod Gadus morhua. Tank experiments on laboratory-bred Atlantic cod (82–141 mm) lasted 150 d. The VIE tag proved to be a good alternative when put on the dorsal part of the fish, with no tag loss and good tag visibility. When put on the ventral part of the fish, however, many VIE tags were covered by growing silvery scales. Retention of soft VIAlpha tags injected behind the eye was quite low (67%), but larger fish had higher tag retention than smaller fish. Survival of experimental fish was high (96%). Mean specific growth rates ranged from 0.46% to 0.51% per day. Both VIE and soft VIAlpha tags were used to individually tag age-0 Atlantic cod (85–290 mm) in the wild. Skin pigmentation of wild Atlantic cod made reading soft VIAlpha tags difficult, while VIE tagging on the dorsal part of the fish still worked well. We suggest that VIE tags are a good alternative for short-term demographic studies on age-0 Atlantic cod, while low retention rates cast doubt on the use of soft VIAlpha tags on these small fish.